Process and apparatus for drying wet particulate material to a desired moisture content



Sept. 7, 1965 T. s. WHITSEL, JR 3,204,341

ING WET PARTICULATE PROCESS AND APPARATUS FOR DRY MATERIAL TO A DESIREDMOISTURE CONTENT 2 Sheets-Sheet 1 Filed NOV. 27, 1961 O 3 0 I5 E m R 2 W5 A 4 R I M 7 O 2 FM; l 3 T 5 2 T 5 E 3 u N P 0 2 3 2 2 m -5 m 2 O WQ 1m3 ZWW 2 n U w m w m w w w w 7 6 w 4 w 2 I 0 DRYING TIME, MINUTES 7O 8OESTIMATED POSITION OF PELLETS IN FULL SCALE DRYER. %OF TOTAL DRYERLENGTH.

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PROCESS AND APPARATUS FOR DRYING WET PARTICULA'I'E MATERIAL To A DESIREDMOISTURE CONTENT Filed Nov. 27, 1961 2 Sheets-Sheet 2 TYPICAL DRYINGcuRvE OF A POROUS soun 70 DRYER LENGTH Emf l-C THERMOCOUPLES Ref. 32F

5,22 6 mv. total DPAP TEMPERATURE "F I713 attorney United States Patent3,204,341 PROCESS AND APPARATUS FOR DRYHNG WET PARTICULATE MATERIAL TO ADESIRED MOISTURE CONTENT Travis S. Whitsel, .llr., Houston, Tex.,assignor, by mesne assignments, to Ashland Gil & Refining Company,Ashland, Ky., a corporation of Kentucky Filed Nov. 27, 1961, Ser. No.155,026 4 Claims. (Cl. 34-39) This invention relates to the continuousdrying of particulate solids. More particularly, it relates to thecontinuous drying of pelletized carbon black. Still more particularly,it relates to an improved method and system for accurately controllingthe heat input to a dryer employed for the continuous drying ofpelletized carbon black.

Carbon black, being finely divided, is usually subjected topelletization as an integral part of its manufacture. For this purpose,either a wet or a dry process is employed. The wet process comprisesadding water to carbon black and subjecting the mass to a mixing actionin suitably designed pelletizers whereby the carbon black is caused toagglomerate into small pellets. The resulting wet pellets are thencontinuously charged into the up stream end of a long, rotary, drum-typedryer in which they are subjected to a temperature sufficiently high todrive off the water, preferably to a moisture content of about 0.11.0%.

Because of variables involved in the drying operation, it has heretoforeproved difficult to continuously produce pelleted carbon black by thewet process having a substantially uniform moisture content. In aneffort to overcome this problem, various automatic control systems havebeen proposed to properly adjust heat requirements to the dryer. Asillustrative of such proposals, for instance, it has been suggested thatadjustments in heat requirements be made in response to variations inthe rate of water introduction to the carbon black pelletizer. Anothersuggested control system is based on adjusting heat requirements inresponse to changes in the temperature of the dried carbon black pelletsleaving the dryer. Another type of suggested control system governsadjustments in heat requirements in accordance with the temperature ofthe exhaust gas in the dryer stack. A still further suggested systemmakes use of dryer stack exhaust gas temperature to control heatrequirement adjustments to the upstream portion of the dryer and thetempera ture of carbon black pellets discharged from the drier tocontrol the remaining portion. These various suggestions, however, havenot proved to be completely satisfactory since they have failed to takeinto account all of the various problems that plague continuous dryingsystems of the type here involved. Among these may be mentioned thelengthy period of time during which the pellets must necessarily remainin the dryer. An additional problem resides in the heat absorptioncharacteristics of the dryer which complicates the relationships betweenfuel input and drying efficiency. Finally, the rate at which wet carbonblack is charged to the upstream end of the dryer is subject to widefluctuations.

The drying of wet pelleted carbon black as it travels through a dryerincludes three phases. In the first phase the surface temperature of thepellets rises through heat exchange with the inside surface of thedryer, with the hot vapors passing through the drum and with otherpellets that are hotter than itself.

In the second phase, the surface temperature of the pellets remains at asubstantially constant value. The temperature remains substantiallyconstant so long as the pellets are coated with moisture and theinternal moisture can diffuse to the surface fast enough to replaceevapo- Patented Sept. 7, 1965 rated surface moisture. Eventually thediffusion rate of the internally held water is not rapid enough toreplace the evaporating surface moisture, with the result that thesurface moisture disappears and the temperature of the pellets againbegins to rise. The location within the dryer where the average surfacetemperature of the pellets first barely exceeds the aforesaid constantvalue is the temperature breaking point, or TBP. The TBP marks the endof the second phase of the drying process and its exact position withinthe dryer tends to move back and forth as the flow of pelletsfluctuates.

During the third phase, the pellets again increase in surfacetemperature towards the temperature of the dryer itself. During thisthird phase, evaporation takes place beneath the surface of the pellet.Since the internal moisture content of the pellet must diffuse withinthe pellet to the plane where evaporation is taking place, and since thevapors resulting from said exaporation must escape from the pellet alsoby diffusion, the rate of evaporation is limited by the rates ofdiffusion of Water and steam through the pellet. As the plane ofevaporation retreats further toward the center of the pellet, thesurface temperature of the pellet continues to get higher because of theincreased thermal gradient required to force heat from the surface tothe receding plane of evaporation. This thermal gradient is defined interms of temperature increase of the pellet per unit of dryer lengthtraveled.

Under the conditions usually found in this third heating zone, a fairlystable temperature gradient may be achieved. This can be determined inthe laboratory under conditions approximately those found in a fullscale dryer by agitating wet pelleted carbon black of known watercontent in a heated vessel for 70 minutes. As the water evaporates, itis collected and the percentage of water removed periodicallycalculated. This latter is then plotted against drying time as shown inFIGURE 1 of the drawing. Each time the removed water content iscalculated, the temperature of the carbon black bed is also determined.Also plotted in FIGURE 1 is a second abscissa showing the estimatingposition of pellets in a full scale dryer based on an assumed retentiontime of 60 minutes and a discharge temperature of 365 F. The curve ofFIGURE 2 is obtained by plotting temperature against dryer length. TheTBP, i.e., the end of the second, substantially constant materialsurface temperature drying phase, occurs at approximately 33% of dryerlength and is indicated in FIGURE 2 as point A. The TBP, i.e., the pointin the dryer at which the surface temperature barely exceeds the dewpoint, occurs at approximately 33% of dryer length and is indicated inFIGURE 2 as point iA'7,

The results of the above laboratory determination, as illustrated by thecurves of FIGURES 1 and 2, indicate that about 67% of the water contentof the pelleted carbon black has been evaporated by the time the TBP hasbeen reached. Thus, approximately two-thirds of the drying is completedwithin approximately the first third of the dryer length. During thenext third of the dryer length, about an additional 27% of the originalwater content is removed. The TBP, therefore, is located atapproximately the mid-point of that portion of the dryer in which 94% ofthe water is removed and is also at the beginning of the previouslydiscussed third heating zone in which the temperature gradient for thelatter stages of drying is established. Movement of the TBP forward andrearward in the dryer will cause variations in the final temperature andmoisture content of the discharged pellets.

The improved method and control system of this invention are based onthe observation that if the TBP can be maintained at a preselected ideallocation so that the slope of the temperature gradient curve for thethird drying zone is maintained substantially constant, the moisturecontent and temperature of the discharged pellets will, in turn, becorrespondingly substantially constant. However, inasmuch as the TBP asa practical matter will move forward and rearward because offluctuations in pellet feed rate and other variables, the objects ofthis invention are met by making appropriate variations in heat inputrequirements to the dryer in response to deviations of the actual TBPfrom a preselected point to compensate for the fluctuations causing suchdeviations and thus maintain the TBP as close as possible to thepreselected point. In this manner, a substantially stable temperaturegradient curve can be maintained and a dried pellet product of uniformmoisture content obtained.

The process and control system of the present invention may be moreadequately described by reference to FIG- URES 3 and 4. FIGURE 3 shows adryer comprising a drum 10, rotatably supported upon Wheels 37 and 38and provided with manifold 11 and conduit 44 communicating with a supplyof wet carbon black pellets. The drum constitutes a drying chamberhaving an inlet and anoutlet and providing a flow path for acontinuously flowing bed 17 of pellets which gradually flows towardsdrum end 12 to discharge over drum lip 18 and through spout 19. Heat issupplied to drum 10 by burner assemblies 13, 14 and 15 communicatingthrough conduit 16 with a source of fuel. Moisture-laden air or recycledcombustion gas or a mixture thereof is introduced through blower 42 andconduit 43 into drum 10 to be discharged through conduit 41 and suctionfan 39.

Provided within drum 10 is a plurality of series connected thermocouples21-32 bracketed about a preselected theoretically ideal location for theTBP indicated by point A in FIGURE 4. In order to provide optimumcontrol in accordance with the process of this invention, the bracketshould preferably be mounted in drum 10 so that point A is halfwaybetween end thermocouples 21 and 32. The process, however, is stilloperative whether or not this situation prevails so that when the termbracketed about point A is used herein it is meant that the group ofthermocouples is mounted so that at least one but preferably half ofthem is on one side of point A and at least one but preferably half ofthem is on the other side of point A. The thermocouples are supportedfrom member in a manner to permit their sensing points to be in constantcontact with or actually slightly submerged beneath the surface ofpellet bed 17. These thermocouples, preferably uniformly spaced, areoperatively connected through control device 33 and conduit 35 topneumatic motor valve 34 in fuel conduit 16. Control device 33 may beany of the known types capable of translating a deviation from astandard into a pneumatic corrective signal. However, a controllerhaving a proportional band control, variable reset rate and rate actionor derivative control is preferable. Although the controller isillustrated herein as having pneumatic signalling, any other type ofsignalling such as electrical, mechanical, hydraulic, or

the like is quite acceptable so long as a valve of corresponding type issubstituted for the pneumatic motor valve 34. A transmitter may or maynot be located between the thermocouples and the controller dependingupon the type of controller.

Since the position of point A will probably vary from dryer to dryer, itis necessary to be able to readily determine this point for each dryer.According to this invention, this may be accomplished by placing thedryer in operation with a temporary group of independently recordingthermocouples suspended at arbitrarily selected points along the lengththereof. No particular number of such thermocouples is required,although the greater the number and the greater resultant span of dryerlength covered, the greater will be the probability of more accuratelylocating point A during the first trial. Each vthernrwcouple should bein contact with the bed and spaced at intervals of 6-18 inches alongabout 10% or more of the length of the dryer. The chief distinctionbetween the permanently mounted thermocouples and the temporary group isthat the latter are not connected in series. Instead, each temporarythermocouple is provided with its own or temperature indicator at alocation outside the dryer.

Once the temporary thermocouples are positioned, the dryer is put intooperation for a period of time sufl-lcient to bring about reasonablystable operating conditions. The input of wet pelleted carbon black aswell as other variables are maintained as close as possible to theconditions which will produce a product of a substantially preselectedmoisture content. Under such conditions, the TBP is stabilized in itstheoretically ideal position. The theoretically ideal position of theTBP is, be definition, point A. The TBP-is also, by definition, thelocation in the dryer at which the second (constant material surfacetemperature) drying stage ends. Thus, temperature readings are takenfrom all of the thermocouples and, if necessary, the group ofthermocouples is moved closer to either end of the dryer and additionalreadings are taken until the location of the TBP is found. This locationis point A. The temporary thermocouples are then replaced by the bracketof thermocouples 21-32, the latter being positioned to bracketestablished point A.

Referring to FIGURES 3 and 4 relative to a description of the operationof the process and system of this invention, it can be seen that theabscissa of the graph of FIG- URE 4 has been placed directly above thedryer drum 10 in FIGURE 3 and has been made of equal length thereto inorder to correlate the location of the thermocouples 21-32 in FIGURE 3with the location of point A in FIGURE 4. For purposes of thisdescription, it should be assumed that the thermocouples are of theIron-Constantin type and that the reference temperature is 32 F. Itshould also be assumed that point A is at a point located at a distanceof approximately one-third of the length of the dryer from the upstreamend thereof.

When the dryer is operating with a steady input, thermocouples 21-32will register the E.M.F.s set forth at the ends of the reference linespointing upwardly therefrom. The total under these assumed conditionswill be 64.74. If the controller 33 is properly calibrated with themotor valve 34, the burners should be delivering heat at the correctrate to maintain the conditions shown by the graph in FIGURE 4. If,however, the input rate increases, the heat input will no longer beadequate to produce a product of uniform moisture content. Accordingly,the actual TBP will move away from point A toward the outlet end ofdryer 10, thus changing the slope of the curve of FIGURE 4 in the regionof the thermocouples and resulting in a lower total In response thereto,the controller 33 will increase the signal transmitted to valve 34 whichwill allow a faster flow of fuel to the burners until such time as theslope returns to normal within the thermocouple region and the TBPreturns to its normal position of coincidence with point A. A similartype of action follows if the input rate diminishes, except that all thevarious factors mentioned above will be reversed to produce .adiminished flow of fuel until the TBP returns to its normal position.

Theoretically, two thermocouples bracketed about point A would besufficient to do the job of sensing the movement of the TBP andcontrolling the gas flow. From a practical point of view, however, alarger number of thermocouples is desirable, since the deviation isthereby magnified for a given extent of movement away from point A. Itshould be understood that the length of the span of thermocouples may beincreased or decreased considerably. In fact, the invention may beoperated successfully with a bracket of thermocouples which spans theentire length of the drum interior. The minimum length of the span mayreadily be determined for any particular dryer by one skilled in theart. It should, of course, be sufficiently great so that movement of theTBP will not exceed the limits thereof. There will invariably be a delaybetween the time a change in heat input is established by the controllerand the time the change is effected. The proportional band mode on thecontroller which adjusts the corrective action in proportion todeviation, and the rate action or derivative mode which further adjuststhe corrective action in proportion to the rate of deviation are veryuseful in compensating the time lag.

I claim:

1. In a process for the drying of wet particles bearing surface andinternal moisture, which particles, when traveling through a dryerprovided with an inlet, an outlet and heating means, rise in surfacetemperature during a first drying phase, remain at a substantiallyconstant surface temperature during a second drying phase and again risestill further in surface temperature drying at third drying phase, thelocation in said dryer where said second, constant surface temperaturephase ends being known as a TBP, temperature breaking point, said TBPbeing apt to move towards said inlet and outlet as the flow of wetparticulate material through the dryer fluctuates, the improvement whichcomprises: varying the heat inputto said dryer in response to movementsof the TBP for maintaining the TBP as close as possible to apredetermined point, said heat input being increased in response tomovements of the TBP toward the dryer outlet and decreased in responseto movements of the TBP toward the dryer inlet, whereby a substantiallyuniform temperature gradient may be maintained in the third drying phaseand a product of correspondingly substantially uniform moisture contentmay be obtained.

2. A process according to claim 1, wherein said particles are wet carbonblack pellets.

3. A dryer system comprising: a rotary chamber having an inlet and anoutlet for wet particulate material, said rotary chamber providing aflow path for said material; burner means associated with said rotarychamber for heating it; a conduit connected with said burner means forsupplying fuel for combustion therein; valve means in said conduit forcontrolling the flow of fuel through said conduit; a plurality oftemperature sensing means mounted within said rotary chamber atlongitudinally spaced points along said flow path for at least about 10percent of the length of said rotary chamber for measuring thetemperature of said particulate material at each of said points, saidtemperature sensing means generating signals representative of the sumof temperatures at the aforesaid spaced points; and means, responsive tosaid signals and connected with said valve and said temperature sensingmeans, for controlling said valve and the heat output of said burner.

4. Apparatus according to claim 3 wherein the temperature sensing meansare thermocouples connected in series and the signals produced therebyare the totals of the E.M.F.s produced by the thermocouples.

References Cited by the Examiner UNITED STATES PATENTS 1,876,025 9/32Sallee 3448 X 2,349,300 5/44 Olsen 3448 X 2,874,482 2/59 Haltmeier 3448X 3,013,785 12/61 King 34-48 X NORMAN YUDKOFF, Primary Examiner.

CHARLES OCONNELL, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 204,341 September 7 1965 Travis S. Whitsel, Jr.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

Column 1 line 57, for "relationships" read relationship column 2, line18, for "exaporation" read evaporation lines 47 to 51, strike out "TheTBP, i.e., the point in the dryer at which the surface temperaturebarely exceeds 4 the dew point, occurs at approximately 33% of dryerlength and is indicated in FIGURE 2 as point "A"."; column 5, line 19,for "drying" read during Signed and sealed this 10th day of May 1966.

(SEAL) Attcst:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. IN A PROCESS FOR THE DRYING OF WET PARTICLES BEARING SURFACE ANDINTERNAL MOISTURE, WHICH PARTICLES, WHEN TRAVELING THROUGH A DRYERPROVIDED WITH AN INLET, AN OUTLET AND HEATING MEANS, RISE IN SURFACETEMPERATURE DURING A FIRST DRYING PHASE REMAINING AT A SUBSTANTIALLYCONSTANT SURFACE TEMPERATURE DURING A SECOND DRYING PHASE AND AGAIN RISESTILL FURTHER IN SURFACE TEMPERATURE DRYING A THIRD DRYING PHASE, THELOCATION IN SAID DRYER WHERE SAID SECOND, CONSTANT SURFACE TEMPERATUREPHASE ENDS BEING KNOWN AS TBP, TEMPERATURE BREAKING POINT, SAID TBPBEING APT TO MOVE TOWARDS SAID INLET AND OUTLET AS THE FLOW OF WETPARTICULATE MATERIAL THROUGH THE DRYER FLUCTUATES, THE IMPROVEMENT WHICHCOMPRISES: VARYING THE HEAT INPUT TO SAID DRYER IN RESPONSE TO MOVEMENTSOF THE TBP FOR MAINTAINING THE TBP AS CLOSE AS POSSIBLE TO APREDETERMINED POINT, SAID HEAT INPUT BEING INCREASED IN RESPONSE TOMOVEMENTS OF THE TBP TOWARD THE DRYER OUTLET AND DECREASED IN RESPONSETO MOVEMENTS OF THE TBP TOWARD THE DRYER INLET, WHEREBY A SUBSTANTIALLYUNIFORM TEMPERATURE GRADIENT MAY BE MAINTAINED IN THE THIRD DRYING PHASEAND A PRODUCT OF CORRESPONDINGLY SUBSTANTIALLY UNIFORM MOISTURE CONTENTMAY BE OBTAINED.